CONDUCTIVITY OF SKIN IN DETERGENT SOLUTIONS 69 to the fact that conductivity across the skin in the SLS/skin/0.9% NaCI system is mainly controlled by sodium and chloride ions. Therefore, the shift arising from the formation of micelies in the SLS/skin/SLS system disappeared. EFFECT OF SLS CONCENTRATION ON BARRIER EFFECT Figure 3 shows the effect of SLS concentration on the resistance of skin treated with cream D. A dimensionless normalized resistance profile, defined as: Rnormalized = Rt/R c (5) O3% SLS 0.• % SLS 0.5 % S LS 0.2 % SLS 'Hme(min) Figure 3. The effect of sodium lauryl sulfate (SLS) concentration on the resistance of skin after treatement with cream D. (.) 0.1%, (•) 0.2%, (') 0.3%, (B) 0.4%, and (O) 0.5%.

70 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS where R t and Rc represent respectively the resistance of skin treated with and without cream, was used as the measurement of the change of resistance with the passage of time. This expression has taken into consideration the baseline difference from various control systems. The normalized resistance approaches unity as the barrier cream is totally washed off. The barrier efficiency of an applied cream decreased with increasing SLS concentration in the donor solution. However, the dependence of the barrier effi- ciency on SLS concentration diminished at SLS concentrations beyond the CMC (0.30% w/w). When the SLS concentration was greater than 0.30% w/w, the barrier effect provided by cream D completely disappeared within 36 min (Figure 3). WHFFE PETROLATUI',4 HYDROFH ILIC PETROLATUM 2 HIyDROPH i LIC OINTMENT 12 24 Ti me ( rain ) Figure 4. Skin resistance after treatment with barrier creams. 0.5% SLS was present in the donor com- partment. (•) white petrolatum, (0) hydrophilic petrolatum, (.) hydrophilic ointment, and (I,) PEG ointment.